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POWhy is an ExpandoObject breaking code that otherwise works just fine?
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copied!<p>Here's the setup: I have an Open Source project called "Massive" (github/robconery/massive) and I'm slinging around dynamics as a way of creating SQL on the fly, and dynamic result sets on the fly.</p> <p>To do the database end of things I'm using System.Data.Common and the ProviderFactory stuff. Here's a sample that works just fine (it's static so you can run in a Console):</p> <pre><code> static DbCommand CreateCommand(string sql) { return DbProviderFactories.GetFactory("System.Data.SqlClient") .CreateCommand(); } static DbConnection OpenConnection() { return DbProviderFactories.GetFactory("System.Data.SqlClient") .CreateConnection(); } public static dynamic DynamicWeirdness() { using (var conn = OpenConnection()) { var cmd = CreateCommand("SELECT * FROM Products"); cmd.Connection = conn; } Console.WriteLine("It worked!"); Console.Read(); return null; } </code></pre> <p>The result of running this code is <strong>"It worked!"</strong></p> <p>Now, if I change the string argument to dynamic - specifically an ExpandoObject (pretend that there's a routine somewhere that crunches the Expando into SQL) - a weird error is thrown. Here's the code:</p> <p><img src="https://i.stack.imgur.com/rHQBk.png" alt="Dynamic Error"></p> <p>What worked before now fails with a message that makes no sense. A SqlConnection <em>is</em> a DbConnection - moreover if you mouseover the code in debug, you can see that the types are all SQL types. "conn" is a SqlConnection, "cmd" is a SqlCommand. </p> <p>This error makes utterly no sense - but more importantly it's cause by the presence of an ExpandoObject that doesn't touch any of the implementation code. The differences between the two routines are: 1 - I've changed the argument in CreateCommand() to accept "dynamic" instead of string 2 - I've created an ExpandoObject and set a property.</p> <p>It gets weirder.</p> <p>If simply use a string instead of the ExpandoObject - it all works just fine!</p> <pre><code> //THIS WORKS static DbCommand CreateCommand(dynamic item) { return DbProviderFactories.GetFactory("System.Data.SqlClient").CreateCommand(); } static DbConnection OpenConnection() { return DbProviderFactories.GetFactory("System.Data.SqlClient").CreateConnection(); } public static dynamic DynamicWeirdness() { dynamic ex = new ExpandoObject(); ex.TableName = "Products"; using (var conn = OpenConnection()) { //use a string instead of the Expando var cmd = CreateCommand("HI THERE"); cmd.Connection = conn; } Console.WriteLine("It worked!"); Console.Read(); return null; } </code></pre> <p>If I swap out the argument for CreateCommand() to be my ExpandoObject ("ex") - it causes <em>all</em> of the code to be a "dynamic expression" which is evaluated at runtime.</p> <p>It appears that the runtime evaluation of this code is different than compile-time evaluation... which makes no sense.</p> <p>**EDIT: I should add here that if I hard-code <em>everything</em> to use SqlConnection and SqlCommand explicitly, it works :) - here's an image of what I mean:</p> <p><img src="https://i.stack.imgur.com/gJu4w.png" alt="enter image description here"></p>

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